The present invention relates to early and non-invasive diagnosis of the depth and severity of skin burns. Such burns, when severe, are not only scarring and crippling, but also have been described as among the most painful injuries that a person can endure. Accurate estimation of the depth and severity of dermal burns would greatly add to the ability of the medical community to appropriately diagnose and treat such injuries.
Each year about two million Americans suffer serious burns. A large number of them require hospital treatment and 10,000-12,000 die from their injuries. Among those hospitalized, some 70,000 people require intensive care, and the cost of such treatment runs several hundred million dollars a year.
Serious burns are complex injuries. In addition to the burn injury itself, a number of other functions may be affected. Bum injuries can affect muscles, bones, nerves, and blood vessels. The respiratory system can be damaged, with possible airway obstruction, respiratory failure and respiratory arrest. Since burns injure the skin, they impair the body""s normal fluid/electrolyte balance, body temperature, body thermal regulation, joint function, manual dexterity, and physical appearance. This damage often restricts the ability of the immune system to protect the burned area, resulting in extreme danger of infection and increasing the extent of permanent scarring. In addition to the physical damage caused by burns, patients also may suffer emotional and psychological problems that begin at the emergency scene and could last a long time.
Skin burns come in varying severity. The mildest, a first degree burn, is a superficial injury that involves only the epidermis or outer layer of skin. The skin is reddened and can be extremely painful, a sign that the nerves retain their function. Such a burn will heal on its own without scarring within two to five days. There may be peeling of the skin and some temporary discoloration.
More serious are second degree burns, in which the first layer of skin is charred through. The second layer, the dermal layer, is damaged in such a burn, but the burn does not pass through to underlying tissues. The skin appears moist and there will be deep intense pain, reddening, blisters and a mottled appearance to the skin. Second degree burns are considered minor if they involve less than 15 percent of the body surface in adults and less than 10 percent in children. When treated with reasonable care, second degree burns will heal themselves and produce very little scarring. Healing is usually complete within three weeks.
Third degree burns are the most severe, and involve all the layers of the skin. They are referred to as full thickness burns and are the most serious of all burns. These are usually charred black and include areas that are dry and white. While a third-degree burn may be very painful, some patients feel little or no pain because the nerve endings have been destroyed. This type of burn may require skin grafting. As third degree burns heal, dense crippling scars form.
The depth of a burn is the critical factor in the diagnosis and treatment of second and third degree burns. The most common treatment for moderate burns is to allow the natural sloughing of dead tissue, and then allow new skin to grow while protecting the burned region with moist bandages. In the case of burns serious enough that some thickness of dermis is actually destroyed, however, it can take as much as two weeks to determine if the residual dermis is capable of satisfactory regrowth, or if skin grafts must be carried out. The primary unknown here is the depth of the seriously damaged region. It is therefore important to be able to measure the extent of dead tissue soon after the burn occurs, and then to treat it promptly with the appropriate technique.
Previous methods to assess bulk depth were subjective and prone to error. Such methods have included, first and foremost, the detailed visual examination of an experienced medical practitioner. An early method to assess bulk depth is simply to biopsy the affected tissues. This invasive procedure, however, is painful, causes additional damage to already damaged tissues, and increases the very real danger of life-threatening infection.
A number of minimally invasive procedures to help in diagnosis of skin burns have been proposed, mostly aimed at detecting the amount of blood circulation remaining in the affected tissues. These include topical application of methylene blue, injection of fluorescein and its detection in the burned region by ultraviolet induced fluorescence, laser Doppler flowmetry, and fluorescence of intravenously injected indocyanine green dye. Such methods were found to be unable to accurately determine burn depth, and therefore have not enjoyed widespread acceptance in the medical community.
A NASA invention involved the use of ultrasound examination to estimate burn depth. When skin is burned, a constituent protein, collagen, becomes more dense. If there is an abrupt interface between burned and unburned tissues as depth is increased, this change of density produces a reflection of an input ultrasonic wave which can indicate burn density. Unfortunately, this interface is rarely abrupt, and hence the technique does not reliably measure burn depth. In addition, a sound transmission medium (typically an organic gel) is usually needed to couple the ultrasonic wave into the burned tissue. Such contamination of the burned tissue is contraindicated in virtually all treatment modalities. Accordingly, this technique has also met with little success (although it is FDA approved for burn diagnosis).
Several approaches to the use of characteristic thermal radiation from burned tissue have been proposed. Time-independent thermal imaging has been used in attempts to assess burn wounds. This approach is based on the assumption that (long wavelength) infrared emission from the skin is related to the amount of cutaneous blood flow in the surface of the wound. This assumption has been found to be wanting. The infrared emission from partial thickness burns is more closely related to heat sources in the underlying, unburned tissues than to the surface blood flow. In addition, such techniques suffer from variation of even normal skin temperature among the patient population, the cooling or heating effects of the immediate environment, the variability in emissivity of burned skin, and a number of other effects. Between the incorrect model and these uncharacterized biasing effects, such time-independent thermal imaging is rendered confusing at best and misleading at worst as an aid to burn diagnosis.
There is a need for improved diagnostic techniques for the depth of skin burns to accurately and quickly guide appropriate medical treatment. There is an additional need for such a technique to be minimally or non-invasive. Ideally, the apparatus for such a technique would be portable, rugged, and affordable, so as to be suitable for field triage and emergency room use.
The present invention measures skin surface thermal relaxation time constants to determine skin burn depth. In one implementation, a thermal camera attached to a video recorder (or the digital equivalent, a frame buffer which converts a video signal to a string of digital data and a digital data recorder, such as a hard disk, magnetic tape, other mass memory devices, or random-access semiconductor memory) records the time-dependent response of the skin to heating or cooling by a small amount (typically a few degrees of temperature). The thermal stimulus can be delivered by a heat lamp, hot or cold air, or other means. Combining information from early and late thermal images reveals areas of the skin which return to equilibrium temperature at different rates, which correspond to different burn depths. As the time scale for return to equilibrium is typically less than a second, variation of external conditions during the examination do not generally affect this measurement technique.